Thin oxide coatings are widely used in industry, in particular in optics, as protective coatings or to fulfil optical functions. Thus, they can serve as protection against corrosion and mechanical damage or for anti-reflection coating of the surfaces of optical components and instruments such as, in particular, lenses, mirrors, prisms, etc. Furthermore, thin oxide coatings are used to produce optical coatings of high, medium or low refractive index in order to increase or reduce reflections. The major areas of application are the production of anti-reflection coatings on spectacle lenses and on elements for camera lenses, binoculars and optical components for optical measuring instruments and for laser technology. Other applications involve the production of coatings having a certain refractive index and/or certain optical absorption properties, for interference mirrors, beam dividers, heat filters and diathermic mirrors, for example.
The starting materials for the production of oxide coatings of this type are known per se. The usual materials are SiO.sub.2 and a wide range of metal oxides, optionally in combination with one another. Selection is made essentially empirically in accordance with the desired optical properties and processing properties. The coatings are produced by the vacuum vapor deposition technique, which is know per se. An exemplary illustration is given in German Patent 12 28 489 and by H. A. Macleod in "Thin Film Optical Filters, A. Hilger Ltd., Bristol, 1986, which comments on the materials that can be used, processing techniques and the problems encountered.
For the production of coatings of medium refractive index, i.e., coatings which have optical refractive index values of 1.6-1.9, the choice of starting materials which are suitable in principle is limited. Suitable starting materials are essentially the oxides of aluminum, magnesium, yttrium, lanthanum, praseodymium, and cerium fluoride and lanthanum fluoride, and mixed systems thereof. The preferred starting material for coatings of medium refractive index is aluminum oxide.
However, these suitable materials have a number of disadvantages which are evident, in particular, from the practical point of view during processing.
One aspect here is that these substances have high melting and boiling points, which are relatively close to one another. From a practical point of view, however, it is important that the vapor-deposition materials are fully melted before significant deposition begins. Only then is a uniform and adequate deposition rate ensured. This is necessary for the formation of homogeneous and uniformly thick coatings on the objects to be coated. However, such requirements are not met under practical application conditions for the oxides of magnesium and yttrium. These substances do not fully melt or do not melt at all under conventional working conditions. They are difficult to evaporate, and, therefore, coatings having thickness variations are obtained.
Magnesium oxide forms porous coatings into which moisture is easily included, causing the coating to become unstable. The same applies to lanthanum oxide. Cerium fluoride and lanthanum fluoride likewise form inhomogeneous coatings of inadequate hardness and durability.
Attempts have therefore been made to reduce the melting points of the base materials by means of suitable additives. Additives furthermore serve to vary and set the refractive index in the resultant coating within certain limits.
The choice of suitable additives for this purpose is limited by the requirement for freedom from absorption. The only appropriate additives are therefore metal oxides which do not absorb in the near infra-red and in the visible spectral region as far as the near UV wave-length range (to about 200 nm).
Titanium dioxide, praseodymium oxide and neodymium oxide, for example, are unsuitable for this reason.
Although the above mentioned problems can be overcome through a suitable choice of additives or by selecting appropriate mixtures of substances, the use of mixed systems per se in vacuum vapor deposition technology is not preferred. The reason for this is that mixed systems generally evaporate incongruently, i.e., they change their composition during the evaporation process, and the composition of the deposited coatings and thus their refractive index also vary correspondingly. Typical examples of systems exhibiting this disadvantage are tantalum oxide/aluminum oxide and hafnium oxide/aluminum oxide mixed systems.